1. Field of the Invention
The present invention relates to an electrophotographic toner and an electrophotographic developer capable of being used in an electrophotographic apparatus (image forming apparatus) utilizing an electrophotographic process, such as a duplicator, a printer and a facsimile machine. The invention also relates to an electrophotographic toner, a process for producing the same, and a process for forming an image.
2. Description of the Related Art
A process of visualizing image information through an electrostatic latent image, such as the electrophotographic process, has been widely used in various fields of art. In the electrophotographic process, an electrostatic latent image is formed on a surface of an electrophotographic photoreceptor (an electrostatic latent image carrying member, which is hereinafter sometimes referred simply to as a photoreceptor) through a charging step and an exposing step, and is then developed with a toner for developing an electrostatic latent image (hereinafter, sometimes referred simply to a toner) to form a developed image, which is then subjected to a transferring step and a fixing step to visualize the electrostatic latent image.
As the process for producing a toner, the kneading and pulverizing method and the emulsion polymerization particle aggregating method have been known. A toner obtained by the pulverizing method has a relatively wide particle size distribution and has an irregular shape, and thus the capability thereof is insufficient.
In the emulsion polymerization particle aggregating method, on the other hand, aggregated particles having a particle diameter corresponding to the toner diameter are formed, and then the aggregated particles are fused and integrated by heating to produce the toner. In this method, the structure of the toner can be freely controlled from the inner layer to the surface layer, whereby more precise particle structure control can be realized (as described, for example, in Japanese Patent No. 3,141,783).
In recent years, the image forming process by electrophotography using the toner and developer techniques are being applied to a part of the printing field owing to progress of the digitalization and colorization and conspicuously practiced in the field of graphic arts including on-demand printing. The field of graphic arts referred herein totally means production of creative printed matters with small number of copies, such as those printed by engraving, and mass production of printed matters by replication, duplication and reproduction of original arts, such as calligraphy and painting, and can be defined as a market targeting business fields and sectors relating to production of those printed matters.
However, as compared to the conventional full-fledged printing, the image forming process involves various problems in capability including image quality, such as color reproduction region, resolution and glossiness, texture, uniformity within one image, and maintenance of image quality upon continuous printing for a long period of time, although it can be characterized by on-demand nature as a plate-less printing. Due to the problems, the image forming process is insufficient for applying to the field of graphic arts where market values of productive properties are pursued.
One of the problems relating to stability of images is discoloration or decoloration of a fixed image upon storing for a long period of time. In the field of graphic arts, in particular, the toner density and the color reproduction region are widely distributed on paper, such as maps and photographs, and the tendency of discoloration or decoloration is conspicuous in such a color region that has a low toner density and uses magenta and yellow colors, such as flesh color.
It is considered this is because the colorant molecule absorbs an ultraviolet ray from sunlight or other illuminations to decompose the colorant molecule. A magenta or yellow pigment has absorbance in a short wavelength range and absorbs light having higher energy than a cyan pigment, whereby the bond of the compound constituting the pigment is broken to cause discoloration or decoloration of the pigment. The tendency is conspicuous in such a color region that is of a natural color and has a low toner density, such as flesh color.
Such a method has been proposed that a compound having an ultraviolet ray absorbing capability is contained in a toner along with a pigment to reduce irradiation of the pigment with an ultraviolet ray. In this method, an ultraviolet ray is absorbed, and the light energy thus absorbed is converted to vibration energy within molecules, whereby affection thereof to the other materials in the toner is suppressed. As the material for absorbing an ultraviolet ray, there have been proposed a method of adding a benzophenone compound (as described, for example, in JP-A-06-148928), a method of adding benzophenone and a hindered amine (as described, for example, in JP-A-06-118684), a method of adding a polymer compound having an organic ultraviolet absorbent bonded thereto through covalent bond (as described, for example, in JP-A-09-080797), a method of adding a photochromic material (as described, for example, in JP-A-2004-061813), and a method of adding a thermochromic colorant (as described, for example, in JP-A-2004-061818).
These methods are favorable in ultraviolet ray absorbing capability, but an ultraviolet absorbent represented by benzophenone is generally poor in heat resistance and is liable to be broken due to heat upon kneading and pulverizing and heat upon polymerization in cases of polymerized toners. The molecular skeleton of the ultraviolet absorbent itself is broken upon storing for a long period of time, and the ultraviolet absorbent, which is colorless in the initial stage, is discolored to change the color of the toner. Furthermore, an ultraviolet absorbent not only absorbs an ultraviolet ray and converts the absorbed light energy to vibration energy within molecules, but also often functions as a photosensitizing agent, which transfers the absorbed light energy to other molecules. The tendency of transferring the light energy is conspicuous particularly in the case where the other materials are present in the vicinity of the ultraviolet absorbent as in the toner, and there are some cases where molecules receiving the light energy emit fluorescent light or phosphorescent light. The luminescent amount of fluorescent light or phosphorescent light is generally small, but cannot be ignored in color reproducibility in a natural color region having a low toner density.
Therefore, such toners of yellow color and magenta color are demanded that suffer no discoloration and decoloration due to sunlight or other illuminations, but it is the current situation that the demand cannot be satisfied.